Why is the width of `~` different than `➜` on iTerm2 when using zsh with spaceship-prompt

Try wrapping your RPROMPT in single quotes. I've seen double quotes cause issues for others with their prompts before.

I would also ditch the precmd > setprompt process for the moment and just set RPROMPT directly to simplify for testing. Start with something like

precmd() {
  ...
}
PROMPT=$'\n$(rb_prompt) in $(directory_name) $(git_dirty)$(need_push)\n› '
RPROMPT='%{$fg_bold[cyan]%}$(todo)%{$reset_color%}'

and refine from there.

If your default system shell is bash, your Terminal should start with it. You can check it on General tab under Terminal Preferences. Should look like the following:

If bash isn't your default shell, you can change it by typing:

chsh -s /bin/bash

To configure iTerm2 with zsh you have to open Preferences and change the command on General tab on your default profile. You have to enter /bin/zsh or whatever shell you want.

To install oh-my-zsh type the following:

sh -c "$(curl -fsSL https://raw.github.com/robbyrussell/oh-my-zsh/master/tools/install.sh)"